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This invention relates to a method for improving injection molding and to pellets for improved injection molding.
One feature of this invention is that it may be used to reduce the plasticizing time of the resin (e.g., polycarbonate or polyestercarbonate), and it may improve the consistency of the plasticizing time. These factors are important for achieving a successful product in short cycle time injection molding operations (e.g., forming optical media such as optical disks).
As stated in U.S. Pat. No. 5,068,065, it is generally desirable in injection molding operations to reduce cycle times to the maximum extent possible. Shorter cycle times lead to higher profit margins because more pieces can be produced in a set period of time.
A typical injection molding machine is shown in FIG. 1. To operate the machine, one adds a plastic resin to the hopper. The resin may be in the form of small resin pellets. The pellets are then fed from the hopper into an extrusion device, which is typically a heated cylinder with a screw inside. The resin pellets are plasticized in the cylinder, and are extruded from an aperture into a mold. The term xe2x80x9cplasticizexe2x80x9d refers to the process of transforming the solid resin pellets into homogenous melted resin. The plasticizing time is the total time required to melt and homogenize the volume of resin necessary to fill the mold for the next cycle. An example of this apparatus is shown in U.S. Pat. No. 5,156,860. The molten resin cools in the mold, and the mold is subsequently opened. If the mold is opened too quickly (i.e., before cooling is complete), the part will be ruined. The plasticizing time must be shorter than the cooling time to ensure that the machine will be ready to refill the mold as soon as the previous part is removed. This is especially important for molding machines which are not equipped with a shut off nozzle because the nozzle can drool if the plasticizing time is too long. Moreover, erroneous plasticizing times may cause defective parts. This problem limits the minimum cycle time.
In the past, it was not recognized that the pellet size would have a significant impact on the plasticizing time because the pellets are melted before the resin is injected into the mold.
Commercially available resin pellets for injection molding generally are roughly cylindrical shaped, and have an average height to width ratio of 0.81 or less.
It would be desirable to modify resin pellets to decrease the cycle time to the extent possible, by making the plasticizing time shorter and more uniform.
It has been discovered that the shape of resin pellets fed into an injection molding machine affects the plasticizing time of the molded article. Specifically, it has been discovered that certain pellet shapes can reduce plasticizing time and improve the yield of an injection molding process by narrowing variability in the plasticizing time.
It has further been discovered that both the total plasticizing time and the variability of plasticizing time in an injection molding machine can be reduced by using resin pellets that have a height to width ratio of between 0.82 and 1.
It has also been discovered that maintaining this ratio is very helpful in reducing cycle time for optical disk molding operations.
It has additionally been discovered that using polycarbonate and/or polyester carbonate pellets having a height to width ratio of between 0.82 and 1 will improve the cycle time and variability in optical disk injection molding operations.